Over five years ago, more smart phones had touch screens than those that did not. During the course of the following five years leading up to the present day, the global market for touch screens has seen a consistent compound annual growth rate in excess of 10% per year. The explosive growth of the market for touch screens is a testament to their incredible utility and ease of use. As humans are able to operate them before they even learn to speak, they represent one of the most intuitive human-machine interfaces developed to date. However, one set of users has not realized the benefits of this new technology. People that are blind, or are visually impaired, can find touch screens difficult to interact with because there is no haptic feedback provided by the interface, and because there is nothing to distinguish the location where input is to be provided besides a visual cue. Although some modifications, such as increasing the zoom on the screen, have been applied to cater to this set of users, it can sometimes still be next to impossible for them to operate a touch screen interface without assistance from another person.
A point of sale (POS) terminal is an example of an application for a human-machine interface where it is important for an individual user to operate the interface without assistance from another party. This is because POS terminals will often need to accept sensitive information from a user. FIG. 1 displays a POS terminal 100 where input data is received in the form of taps and swipes on touch screen 102. The average user will have no difficulty utilizing user interface 101 to enter their personal identification number (PIN) to authorize a transfer of funds using the POS terminal. However, a visually impaired person will have no way to enter their pin on the user interface 101 because they will not know where the numbers are displayed to accept their selection. In these situations, the user will need to provide their PIN to a clerk verbally which provides a disadvantageous security situation in that both the clerk, and potential eavesdroppers, will be able to obtain the user's PIN.